Studies on cell surface antigens of metastatic cells involved the analysis of the T10 sarcoma, originated in an (H-2k times H-2b)F1 hybrid. We found that cells of metastatic origin express both parental haplotypes, whereas those of the local population express only the H-2b. The expression of the H-2k seems to be related to the metastatogenic properties of the cells. Continuing our previous studies on the relevance of susceptibility to NK cells to the metastatic properties of tumor cells, we carried out in vivo selection of 3LL tumor cells resistant to NK activity. We found that this resulted in concomitant selection for increased metastatic properties. Studies on the effects of anesthetic drugs commonly used in surgical practice indicated that surgical excision of the local tumor under these anesthetics resulted in accelerated progression of metastasis. The function of the spleen in controlling the postoperative progesssion of metastasis was investigated, indicating a suppressor effect which was removed following splenectomy. This was deduced from experiments in which splenectomy performed concomitantly with excision of the tumor prevented the otherwise accelerated growth of lung metastasis.